The field of the invention is magnetic resonance imaging, and in particular the invention relates to sensing variation of the B.sub.0 polarizing magnetic field resulting from vibration of the imaging apparatus.
Any nucleus, which possesses a magnetic moment, attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency, known as the Larmor frequency, which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. However, if the substance or tissue is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
MRI is particularly useful as a medical diagnostic tool. However, the ability to create detailed images which clearly depict anatomical features of the patient, depends upon an extremely stable polarizing magnetic field B.sub.0. Mechanical vibration of the MRI system perturbs the polarizing magnetic field, thereby producing artifacts in the resultant magnetic resonance images. If the real-time displacement of the magnetic components could be accurately measured, then mathematical models could be used to estimate and correct the magnetic field variation due to that displacement. For whole body magnets, displacements on the order of a micron generate magnetic field changes of approximately one part per million. Thus a motion sensor for artifact correction must be capable of detecting submicron displacements.
The obvious approach to measuring the vibration would be to sense the variation of the polarizing magnetic field. However, the imaging system produces other magnetic fields, which vary at radio frequencies and thus can adversely affect the ability to sense changes in the polarizing magnetic field.